Cortical visual prosthesis

ABSTRACT

The present invention consists of an implantable device with a hermetic electronics package that houses electronics. The hermetic package is attached to a flexible circuit electrode array having its electrodes arranged in a trapezoidal electrode array field that is suitable to stimulate the visual cortex. The hermetic electronics package is provided with a fixation structure that secures, protects and dissipates heat from the electronics package. The entire implantable device can be entirely implanted within the head.

CROSS REFERENCE TO RELATED APPLICATIONS

This application incorporates by reference and claims priority to U.S.Provisional Patent Application 62/161,134, filed May 13, 2015, forCortical Visual Prosthesis; U.S. Provisional Patent Application62/170,052, filed Jun. 2, 2015, for Cortical Visual Prosthesis; and U.S.Provisional Patent Application 62/327,944, filed Apr. 26, 2016, forCortical Visual Prosthesis.

FIELD OF THE INVENTION

The present invention is an implantable device for interfacing withneural tissue of the visual cortex, primarily in order to induce theperception of light directly to the brain.

SUMMARY OF THE INVENTION

The present invention consists of an implantable device with a hermeticelectronics package that houses electronics. The hermetic package isattached to a flexible circuit electrode array having its electrodesarranged in a trapezoidal electrode array field that is suitable tostimulate the visual cortex. The hermetic electronics package isprovided with a fixation structure that secures, protects and dissipatesheat from the electronics package. The entire implantable device can beentirely implanted within the head.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Shows the preferred package for the present inventionillustrating basic structure and means of attachment and location ofimplantation.

FIG. 2A is a diagram of the invention as implanted for brain penetratingstimulation.

FIG. 2B is a diagram of the invention as implant for brain surfacestimulation.

FIG. 3 is a perspective view of a partially built package showing thesubstrate, chip and the package wall.

FIG. 4 is a perspective view of the hybrid stack placed on top of thechip.

FIG. 5 is a perspective view of the partially built package showing thehybrid stack placed inside.

FIG. 6 is a perspective view of the lid to be welded to the top of thepackage.

FIG. 7 is a view of the completed package attached to an electrodearray.

FIG. 8 is a cross-section of the package.

FIG. 9 is a perspective view showing the outside of the implantableportion of the cortical stimulator

FIG. 10 is a perspective view showing the locations of the electrodesand coil of the implantable portion of the cortical stimulator.

FIG. 11 is a top view of the implantable portion of the corticalstimulator.

FIG. 12 is a bottom view of the implantable portion of the corticalstimulator.

FIG. 13 is a bottom view of the implantable portion of the corticalstimulator excluding the mounting fixture.

FIG. 14 is a bottom perspective view of the implantable portion of thecortical stimulator excluding the mounting fixture.

FIG. 15 is a top perspective view of implantable portion of the corticalstimulator excluding the mounting fixture.

FIG. 16 shows the implantable portion of the cortical stimulator asimplanted in a head.

FIG. 17 is a bottom view of a further alternate embodiment.

FIG. 18 is a top view of a further alternate embodiment.

FIG. 19 is an aggregate electrode according to the present invention.

FIG. 20 is an aggregate electrode with a center sensing electrode.

FIG. 21 is an aggregate electrode with variable size segments.

FIG. 22 is a perspective view of the implant components including aprotective cover plate.

FIG. 23 is a top view of a skull showing the location of the implantableportion of the cortical stimulator.

FIG. 24A is a view of a subject wearing the external portion of thecortical stimulator including glasses with camera and transmitter coil.

FIG. 24B is a perspective view of the video processing unit.

FIG. 25 is an image showing the location of the visual cortex on thebrain.

FIG. 26 is a view of a child or animal version of the implant.

FIG. 27 is an x-ray showing the animal version as implanted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Overall, the present invention has the capability to robustly andreliably stimulate the visual cortex of the brain and be implantedentirely within the head. The preferred embodiment includes a 60 channelstimulator producing a maximum current of 8 mA at a maximum frequency of120 Hz to 50 kHz. The stimulator produces symmetrical biphasicrectangular pulses with an interphase delay (cathodic or anodic first).The pulse width range is 32 μs to 3.3 ms (0.25 ms at 8 mA). Theinterphase delay range is 32 μs to 3.3 ms (total waveform <6.6 ms).Electrode diameter: 2 mm. The wireless link range between the implantand external coils is about 30 mm.

Cortical prostheses of the preferred embodiment will provide thesefeatures: neural recording, programmable stimulation waveforms, and highcurrent output capability. The electrodes will be coated with PlatinumGray as described in U.S. Pat. No. 6,974,533, which is incorporatedherein, allowing for a chronic charge delivery capability of 1 mC/cm².The preferred cortical prosthesis will insure safety though impedancechecks, electrode Integrity checks, waveform (pulse & frame)measurements, continuous monitoring of device Safety (leakage,overheating, catastrophic failure etc.) and continuous monitoring ofdevice reliability.

The preferred cortical prosthesis will provide research tools forlow-level psychophysics experiments. The tools include an extensibleprogramming platform built on top of a psychophysics ApplicationProgramming Interface (API), The platform can be used to design anddevelop low-level psychophysics experiments for subjects (e.g.perception threshold/sensitivity measurements, spatial mapping, dynamicrange, two-point discrimination, chronaxie curves, easy and fastcreation of optimal configuration parameters, frequency of stimulation,pulse parameters, timing groups and additional configurable systems fortesting visual prosthetics.

The preferred embodiment of the present invention is shown in FIG. 1,consists of an electronics package 14 that is preferably oval orcircular in shape (but other shapes are possible) less than 25 mm indiameter (preferably less than 15 mm and more preferably less than 11 mmin diameter), and that is less than 8 mm in height (preferably less than4 mm and more preferably less than 3.5 mm in height) that is mountedwithin a hollowed out portion of the cranium or on top of the craniumbut under the skin. The package may include a feature for mounting tothe cranium such as a low profile flange defining holes to accommodatescrews, or tabs that allow screws, sutures or staples to be taken to fixthe package (see FIG. 9). Attached to and proceeding from the package 14is a thin film lead 10 to be routed to the tissue to be stimulated orrecorded from. The electrode array is implanted on the surface of thevisual cortex. FIG. 1 is a cut away view showing half of the head andbrain. The electrode array is suitable to be placed on the corticalsurface while the electronics package 14 is within the cranium.

FIGS. 2A and 2B show attachment of the package 14 to the cranium 7,Alternatively, the package 14 may be affixed to the cranium through theuse of one or more straps, or the package 14 may be glued to the craniumusing a reversible or non reversible adhesive attach. In this embodimentthe package, which protrudes from the cranium, is low profile and shapedin manner that permits the scalp 8 to rest on top of the package withlittle or no irritation of the scalp. Additionally, edges of the packageare preferably rounded and or the package 14 may be encased in a softpolymer mold such as silicone to further reduce irritation. In otherembodiments the package 14 may be attached to the scalp 8, brain 11 ordura 12. In embodiments with more than one package each package may beattached to any of the scalp, cranium, dura, or brain.

The improved package of the present invention allows for miniaturizationof the package which is particularly useful in brain sensors,stimulators and other prostheses for electrical sensing and stimulationof neural tissue.

The electronics package 14 is electrically coupled to a secondaryinductive coil 16. Preferably the secondary inductive coil 16 is madefrom wound wire. Alternatively, the secondary inductive coil 16 may bemade from a flexible circuit polymer sandwich with wire traces depositedbetween layers of flexible circuit polymer. The electronics package 14and secondary inductive coil 16 are held together by the molded body 18.The molded body 18 holds the electronics package 14 and secondaryinductive coil 16 end to end. This is beneficial as it reduces theheight the entire device rises above the skull surface. The design ofthe electronic package (described below) along with a molded body 18which holds the secondary inductive coil 16 and electronics package 14in the end to end orientation minimizes the thickness or height abovethe skull surface of the entire device.

The Silicone elastomer can be formed in a pre-curved shape to match thecurvature of the skull. However, silicone remains flexible enough toaccommodate implantation and to adapt to variations in the curvature ata particular site. The implant secondary inductive coil 16, whichprovides a means of establishing the inductive link between the externalprocessor (see FIG. 24) and the implanted device, preferably consists ofgold wire. The wire is insulated with a layer of silicone. The secondaryinductive coil 16 may be round or oval shaped. The conductive wires arewound in defined pitches and curvature shape to satisfy both theelectrical functional requirements and the surgical constraints. Thesecondary inductive coil 16, together with the tuning capacitors in thechip 64, forms a parallel resonant tank that is tuned at the carrierfrequency to receive both power and data.

Since the implant device may be implanted just under the scalp it ispossible to irritate or even erode through the scalp. Eroding throughthe scalp leaves the body open to infection. We can do several things tolessen the likelihood of scalp irritation or erosion. First, it isimportant to keep the overall thickness of the implant to a minimum.Even though it may be advantageous to mount both the electronics package14 and the secondary inductive coil 16 on the cranium just under thescalp, the electronics package 14 is mounted at a distance laterallydisplaced from the secondary inductive coil 16. In other words thethickness of the secondary inductive coil 16 and electronics packageshould not be cumulative.

It is also advantageous to place protective material between the implantdevice and the scalp. The protective material can be provided as a flapattached to the implant device or a separate piece placed by the surgeonat the time of implantation. Adding material over the device promoteshealing and sealing of the wound. Suitable materials include Dacron,Teflon (polytetraflouroethylene or PTFE), Goretex (ePTFE), Tutoplast(sterilized sclera), other processed tissue, Mersilene (Polyester) orsilicone. Referring to FIG. 3, the hermetic electronics package 14 iscomposed of a ceramic substrate 60 brazed to a metal case wall 62 whichis enclosed by a laser welded metal lid 84. The metal of the wall 62 andmetal lid 84 may be any biocompatible metal such as Titanium, niobium,platinum, iridium, palladium or combinations of such metals. The ceramicsubstrate is preferably alumina but may include other ceramics such aszirconia. The ceramic substrate 60 includes vias (not shown) made frombiocompatible metal and a ceramic binder using thick-film techniques.The biocompatible metal and ceramic binder is preferably platinum flakesin a ceramic paste or frit which is the ceramic used to make thesubstrate. After the vias have been filled, the substrate 60 is firedand lapped to thickness. The firing process causes the ceramic tovitrify binding the ceramic of the substrate with the ceramic of thepaste forming a hermetic bond. The package wall 62 is brazed to theceramic substrate 60 in a vacuum furnace using a biocompatible brazematerial in the braze joint. Preferably, the braze material is a nickeltitanium alloy. The braze temperature is approximately 1000° Celsius.Therefore the vias and thin film metallization 66 must be selected towithstand this temperature. Also, the electronics must be installedafter brazing. The chip 64 is installed inside the package usingthermocompression flip-chip technology. The chip is underfilled withepoxy to avoid connection failures due to thermal mismatch or vibration.

Referring to FIGS. 4 and 5, off-chip electrical components 70, which mayinclude capacitors, diodes, resistors or inductors (passives), areinstalled on a stack substrate 72 attached to the back of the chip 64,and connections between the stack substrate 72 and ceramic substrate 60are made using gold wire bonds 82. Alternatively discrete components, ora circuit board, may be attached to the ceramic substrate. A flip-chipintegrated circuit and/or hybrid stack is preferred as it minimizes thesize of the package 14. The stack substrate 72 is attached to the chip64 with non-conductive epoxy, and the passives 70 are attached to thestack substrate 72 with conductive epoxy. Thin-film metallization 66 isapplied to both the inside and outside surfaces of the ceramic substrate60 and an ASIC (Application Specific Integrated Circuit) integratedcircuit chip 64 is bonded to the thin film metallization on the insideof the ceramic substrate 60.

Referring to FIG. 6, the electronics package 14 is enclosed by a metallid 84 that, after a vacuum bake-out to remove volatiles and moisture,is attached using laser welding. A getter (moisture absorbent material)may be added after vacuum bake-out and before laser welding of the metallid 84. The metal lid 84 further has a metal lip 86 to protectcomponents from the welding process and further insure a good hermeticseal. The entire package is hermetically encased. Hermeticity of thevias, braze, and the entire package is verified throughout themanufacturing process.

Referring to FIG. 7, the flexible circuit thin film lead 10, includesplatinum conductors 94 insulated from each other and the externalenvironment by a biocompatible dielectric polymer 96, preferablypolyimide. One end of the array contains exposed electrode sites thatare placed in close proximity to the surface to be stimulated orrecorded from. The other end contains bond pads 92 that permitelectrical connection to the electronics package 14. The electronicpackage 14 is attached to the flexible circuit 10 using a flip-chipbumping process, and epoxy underfilled. In the flip-chip bumpingprocess, bumps containing conductive adhesive placed on bond pads 92 andbumps containing conductive adhesive placed on the electronic package 14are aligned and cured to build a conductive connection between the bondpads 92 and the electronic package 14. Leads 76 for the secondaryinductive coil 16 are attached to gold pads 78 on the ceramic substrate60 using thermal compression bonding, and are then covered in epoxy. Thejunction of the secondary inductive coil 16, thin film lead 10, andelectronic package 14 are encapsulated with a silicone overmold 90 thatconnects them together mechanically. When assembled, the hermeticelectronics package 14 may sit an arbitrary distance away from the endof the secondary inductive coil.

Referring to FIG. 8, the package 14 contains a ceramic substrate 60,with metallized vias 65 and thin-film metallization 66. The package 14contains a metal case wall 62 which is connected to the ceramicsubstrate 60 by braze joint 61. On the ceramic substrate 60 an underfill69 is applied. On the underfill 69 an integrated circuit chip 64 ispositioned. On the integrated circuit chip 64 a ceramic hybrid substrate68 is positioned. On the ceramic hybrid substrate 68 passives 70 areplaced. Wirebonds 67 are leading from the ceramic substrate 60 to theceramic hybrid substrate 68. A metal lid 84 is connected to the metalcase wall 62 by laser welded joint 63 whereby the package 14 is sealed.

FIG. 9 shows a perspective view of the preferred embodiment showing theoutside of the implantable portion. FIG. 10 adds the locations of theelectrodes and coil of the implantable portion. Note from this view theelectrodes are show through the flexible circuit electrode array 110.That is the electrodes are on the other side. It is advantageous thatthe flexible circuit electrode array 110 be made in a trapezoidal shapewith the cable portion attached to the smallest side of the trapezoid.This shape better accommodates the target tissue on the medial surfaceof the visual cortex. The molded body 119 holding the electronicspackage 114 and the coil 116 is arranged with the coil 116 opposite theflexible circuit electrode array 110. The device is intended to beimplanted with the flexible circuit electrode array 110 attached on topof the package (toward the outside of the skull). This allows theelectrodes to be on the same side of the flexible circuit electrodearray 110 as the bond pads connecting the flexible circuit electrodearray 110 to the electronics package 114 and still face down toward thebrain. The ceramic substrate portion of the electronics package 114 towhich the flexible circuit electrode array 110 is attached is moredelicate than the metal can portion. A mounting fixture 115 covers andprotects the electronics package 114, provides screw tabs for attachingthe electronics package 114 to the skull and further provides a heatsink to dissipate heat from the electronics package 114 and a returnelectrode. The mounting fixture 115 may be any biocompatible metal suchas titanium, niobium, platinum, iridium, palladium, or an alloy orcombination of such metals. The silicone body 18, described above mayfill, or partially fill, any gaps between the electronics package 114and the mounting fixture 115. The electronics package 114, coil 116 andmolded body 118 are implanted within a hollowed out portion of theskull. it may be necessary to cut through the skull for the electronicspackage 114 but only part way through the skull for the coil 116 whichis not as thick as the electronics package 114. Also, the electronicspackage 114 can be the return electrode, in which case it needs tocontact the Dura to reduce impedance. Only a small slot is needed tofeed the flexible circuit electrode array 110 through to its implantedlocation (see FIG. 16). This provides better protection to the brainthan an implant where large portions of the skull are removed. Theoverall device is preferably about 9.7 cm in length. The electrode arrayportion 110 is preferably about 2.4 cm by 3.4 cm. The coil andelectronics molded body is preferably 1.1 cm or less in width. Eachelectrode is preferably about 2 mm in diameter.

FIG. 11 is a top view of the implantable portion, similar to theperspective views of FIGS. 9 and 10. FIG. 12 provides a bottom view ofthe implantable portion, opposite of the view previously seen. This isthe view that would face the brain when implanted, showing theelectrodes. FIG. 13 is a bottom view of the implantable portionexcluding the mounting fixture to better show the electronics package114. FIG. 14 is a bottom perspective view of the implantable portionexcluding the mounting fixture. FIG. 15 is a top perspective view ofimplantable portion excluding the mounting fixture.

FIG. 16 shows the implantable portion as implanted in a head. The moldedbody 118, coil 116, and hermetic package 114 are placed in a section ofhollowed out skull, preferably not all the way through the skull. Thenthe fixation structure 115 is screwed into the skull. This protects thehermetic package 114 and prevents any movement of the hermetic package114 from being transmitted to the flexible circuit electrode array 110on the brain surface. Only a small slot is required for the flexiblecircuit electrode array 110 to pass through the skull. In most cases thesurgeon will removed a larger area of skull to properly place theflexible circuit electrode array 110. The removed skull is then replacedand the resulting crack is enough to provide for the slot to allow theflexible circuit electrode array to pass through the skull. The rest ofthe skull may then heal around the flexible circuit electrode array 110.

FIG. 17 shows a further alternate embodiment as seen from the bottom ortoward the brain. In this embodiment the coil 116 is at 90 degrees tothe flexible circuit electrode array 110. This embodiment may be used ona smaller skull (such as a child or an animal), or where space isotherwise limited. FIG. 18 shows the alternate embodiment of FIG. 17from the top, or away from the brain.

Referring to FIG. 19, the electrodes shown in FIGS. 10-15, 17 and 18 areaggregate electrodes 120 within electrode array 110 not singleelectrodes. Aggregate electrodes 120 are made up of small sub-electrodes122. The small sub-electrodes 122 are linked by traces 124 to make thesub-electrodes 122 common and forming aggregate electrode 120. In orderto stimulate neurons relatively far from the electrode surface (over 1mm), such as in subdural cortical applications, the aggregate electrode120 diameter needs to be of a similar size. Conventional electrodes area circle, filled entirely of the electrode material. Creating a largethin-film electrode in that manner is prone to problems ofmanufacturability and long-term reliability and plating deposits mayflake off.

The deposition of the platinum gray or other advanced materials oversuch a large area introduces stresses into the material that may lead tocracking or delamination. The adhesion of the metal to the underlyingpolymer will also be poor, creating a high risk of delamination of thelayers.

In this design, the outer diameter of the aggregate electrode 120 islarge (between 1 and 3 mm), but the surface area of electrode materialis reduced. This is achieved through distributing the electrode materialas discrete islands or sub-electrodes 122. The sub-electrodes areshorted together through platinum traces 124 embedded in the underlyingpolymer dielectric. The diameter of each sub-electrode 122 is 200 um,equivalent to the existing electrode diameters, a size in which platinumgray is deposited without creating excessive stress in the material. Thestability and reliability of sub-electrodes 122 of this size has beendemonstrated in lab testing and long-term implantation. The amount ofmetal conductor is also minimized in this design, providing a largerarea for strong polymer interface bonding. While the sub-electrodes arenormally relatively flat, in an alternate embodiment the sub-electrodesare made to protrude somewhat from the electrode array surface, into thecortical tissue to help reduce thresholds. In another alternateembodiment, the entire group of sub-electrodes that forms an aggregateelectrode are protruding slightly together as a group into the corticaltissue; in this case the electrode array surface between and around thesub-electrodes is slightly raised in a gentle mound-like configuration.

Current density is non-uniform over an electrode, and is highest at theedges, termed the ‘edge effect’. This is typically not desirable as thehigh current density has greater potential for damage to the electrodeand/or target neurons. This design significantly increases the totalperimeter length of the electrode 120, offering the opportunity for amore uniform current distribution. The current density distribution canbe further optimized through customized routing and width of theplatinum conductor traces such that center electrodes experience highercurrent flow than the edge electrodes

Referring to FIG. 20, another potential configuration is to use one ormore of the electrodes 126 as a sensing electrode, wired separately fromthe rest of the sub-electrodes 122. This would allow for optimization ofthe sensing electrode diameter, and enables sensing of tissue responsebefore, during and immediately after stimulation.

Referring to FIG. 21, a further configuration is to wire one or morerings of sub-electrodes 122 independently from the others. The aggregateelectrode 120 diameter can therefore be changed through software, whereall rings can be pulsed together to act as a large aggregate electrode120, or by disabling outer rings, could behave as a smaller aggregateelectrode 120. This could be done as a part of patient fitting in theclinic or real-time to allow for dynamic percept size.

Referring to FIG. 22, the implant may also include a cover plate 117.The cover plate protects the implant in the cases of a blow to the head.The cover plate 117 may be any biocompatible metal such as titanium,niobium, platinum, iridium, palladium, or an alloy or combination ofsuch metals. The cover plate 117 may also serve as part of the returnelectrode. The implant consists of: an electrode array 110 and cable; asecondary coil 116 for receiving information and power from the externalcomponents; hermetically-encased electronics 114 to drive stimulation ofthe electrodes; and a cranial socket 115 and cover plate 117.

Initial array dimensions have been determined from published data onfunctional mapping of the visual cortex. The array will be of similarsize to those used by Brindley and Dobelle, so similar visual fieldextents of 20° to 30° are expected; this is similar to the Argus IIvisual field (about 20°). The initial array dimensions are sized to fitwithin the bounds of the visual cortex, at 34 mm long and 24 mm tall ina trapezoidal configuration (as shown in FIGS. 9-15 and 22). Placementof the array may be guided by anatomical landmarks and pre-operativeMRI. Visual cortex location is fairly consistent, and anatomicallandmarks have been shown to be predictive of visual field mapping, soconsistent placement in visual areas is not expected to present anysignificant difficulty. If necessary, fluoroscopy can be used to assistin placement as the electrode array contains two radio-opaque markersnear its extremities.

Various electrode sizes may be employed in a layout such as acheckerboard where every alternate electrode is of a smaller size. Thisapproach was used in several of the first generation Argus implants togather data on the quality of the percepts with respect to electrodesize. The electrodes will be up to 2 mm in diameter, which have beenreported to produce punctate phosphenes generally indistinguishable fromsmaller electrodes while also encompassing enough surface area to allowfor safe charge densities.

The aggregate electrodes are square packed with 3 mm center to centerdistance, a sufficient spacing which should allow for distinct percepts.Sixty electrodes will be used initially so that the existing matureArgus II electronics can be leveraged in this design. The only minorchange to the implantable electronics is to supply more current (8 mA vs1 mA in Argus II), which based on the literature should be adequate forsurface cortical stimulation, and furthermore is still within the safecharge limits described by Shannon and McCreary for these relativelylarge aggregate electrodes.

Direct anchoring to the brain tissue will not be necessary for theelectrode array since this is the current practice with subacute andchronic implantation of subdural electrocorticographic arrays forepilepsy monitoring and responsive neurostimulation, respectively. Thedesign of the novel flexible electrode array will incorporate anelongated, integrated, flexible polymer cable to accommodate movement ofthe brain relative to the dura and skull. To avoid damage to tissue fromsharp polymer edges, the array and cable will be given a soft siliconebumper around its edge, as in the Argus II System.

Referring to FIG. 23, the placement of the internal coil and electronicscase is another key aspect of the design. The electronics for thecortical prostheses will be affixed to the skull. This is similar to theplacement used for the NeuroPace closed loop stimulator for epilepsy,and the first generation of the Argus implant. The electronics packagewill be embedded inside a metal cranial socket. The cranial socketserves a multifold role of protecting the implant, providing anchorpoints to the skull, assisting in dissipating heat generated by theelectronics and providing a large surface area return electrode for thestimulating current.

Referring to FIGS. 24A and 24B, the external components are anabsolutely critical part of the system. They are based on those of theArgus II System (now in its 16th revision), with modifications for thecortical stimulation. As in the Argus II, the main components will bethe Glasses 200 and a Video Processing Unit (VPU) 206. A small,lightweight video camera 202 will be mounted on the glasses. Thetelemetry coils 204 and radio-frequency system for transmitting datafrom the VPU 206 to the implant will be positioned so they rest on theback of the head, in close apposition to the implanted coil 116.

The VPU 206, which is worn on a belt or strap, is used to process theimages from the video camera 202 and convert the images into electricalstimulation commands; these are transmitted wirelessly to the implant.

Referring to FIG. 25, the visual cortex is composed of the striatecortex (V1) and extrastriate areas (V2, V3 and higher visual areas),which all have some exposure on the medial surface of the occipitallobe. V1 is the primary visual cortex, and is the first step in corticalprocessing of visual signals. Like the retina, physical locations in V1are mapped to locations in visual space (retinotopic or visuotopicmapping). V1 starts at the posterior pole of the occipital lobe andfollows the calcarine sulcus anterior along the medial surface of thebrain. On average, 4.6 cm² of V1, 22% of its total surface area, isexposed on the medial surface, while the majority is buried in thecalcarine sulcus (20). The electrode area of the implant will beapproximately 6.5 cm², larger than the exposed V1 area, so someelectrodes will stimulate extrastriate areas V2 and V3. These two areassurround V1 and each have visuotopic mapping to the entire visual field,which allows for the implant to stimulate parts of the field that for V1are buried in the calcarine sulcus. Encouragingly, stimulation of V2 andV3 has been shown to produce single distinct phosphenes, similar to thatobserved with stimulation of V1.

The extent of visual field is known to be one of the most importantfactors in visually-guided mobility. The best way to cover a large partof the visual field with an electrode array is to locate electrodes overa wide area on the medial surface of the visual cortex. The medialsurface can access all but the very central two to three degrees ofvisual field.

The upper and lower visual fields are split by the calcarine sulcus,with the upper visual field located below the sulcus, and the lowerfield above the sulcus (FIG. 25C). The electrode array will span bothsides of the sulcus, to cover upper and lower vision. A single electrodearray will stimulate one hemisphere, which activates the visual field onthe contralateral side. Initially one can implant a single array, so onehalf of the visual field (left or right) will be stimulated. But,cortical visual prosthesis can enable bilateral implants andstimulation. This can be accomplished with a single driver and twoarrays, or two independent implants with driver and array.

During testing sessions, a subject's Video Processing Unit can bewirelessly connected to a laptop or tablet device. Downloadable apps areused to perform psychophysics research—to determine how stimulationparameters affect the appearance and location of phosphenes. The appsallow subjects' VPUs to be custom-programmed for standalone camera use(i.e., with the electrode stimulation pattern being derived from thevideo feed). We have leveraged and built on the Argus II programming andtesting software (currently in its 16th revision).

The measured impedances of the electrodes range between 1 k-2.5 k Ohms.Some variation is observed in the first few weeks post implantationbefore the impedance values stabilize. This variation is typical andbelieved to be caused during the normal body's response to a foreignbody.

A few iterations of mechanical (non-functional) models of the implanthave been designed, manufactured, and tested in the nonclinical setting.Several initial prototypes of the array and implant were used in cadaverdissections to identify possible surgical approaches and to evaluate thearray size and thickness, cable length, and flexibility of materialsneeded for safe implantation.

In addition, both mechanical models and working prototypes of the fullimplant, including array, cable, implant coil, electronics package, andcranial socket were built for testing in animals (see FIG. 26). Theimplant coil and electronics case were replicas of those for use onhumans while the array size was reduced to fit the smaller brain, with acorresponding reduction in number of electrodes. To date five mechanicalmodels and six fully functional prototypes have been implanted in theanimals (FIG. 27). Metallic markers 300, preferably titanium, may beadded to the electrode array to assist in locating the array in an X-rayor CT scan. The stimulating electrodes were placed on the lefthemisphere of the medial occipital cortex. The primary goals of theanimal studies were:

-   1. Assess suitability and safety of the implant procedure, via    surgeon feedback and collection of intraoperative adverse events.-   2. Assess the long-term safety of the implant via collection of    adverse events during the follow-up period.-   3. Assess the biocompatibility of the implant, via analysis of gross    and microhistology.-   4. Demonstrate the long-term functionality of the implant, via    collection of impedance and implant diagnostic measurements    throughout the study.-   5. Evaluate the implant for electrode array migration, via CT scan.-   6. Demonstrate compliance of the implant to international standards    with respect to in-vivo temperature rise of the implant during    activation.-   7. Demonstrate ease of explant and effect of explant surgery on the    animal model-   8. Demonstrate the safety of chronic stimulation on neural tissue

During the course of animal studies conducted thus far, the implantdesign was refined iteratively to improve its performance with respectto the above criteria. The performance of these devices were monitoredregularly by measuring the impedance. The devices are also were used toapply electrical stimulation to the visual cortex of the animals. Whileapplying stimulation current the animals are sedated so the externalcoil can be positioned properly for continuous communication. Under mildsedation it was noted that the animals tended to consistently turn theirheads to the right as soon as stimulation current was applied (as iflooking at something). While it cannot be conclusively stated that theywere experiencing visual percepts, the direction of turning was alwaysto the right, which would be consistent with the field of visualpercepts produced by stimulation of the left visual cortex.

The measured impedances of the electrodes range between 1 k-2.5 k Ohms.Some variation is observed in the first few weeks post implantationbefore the impedance values stabilize. This variation is typical andbelieved to be caused during the normal body's response to a foreignbody. Following long-term (several months) implant, the animal issacrificed and histopathology is performed. To date, the histopathologyhas not shown any significant levels tissue reaction or neuropil.Expected fibrous encapsulation of the device is observed to varyingdegrees, both on the skull mounted electronics package and the flexibleelectrode array. No physiological or behavioral changes were observed inthe animals post-implantation or post-explanation.

Accordingly, what has been shown is an improved cortical visualprosthetic for implantation in a body. While the invention has beendescribed by means of specific embodiments and applications thereof, itis understood that numerous modifications and variations could be madethereto by those skilled in the art without departing from the spiritand scope of the invention. It is therefore to be understood that withinthe scope of the claims, the invention may be practiced otherwise thanas specifically described herein.

What we claim is:
 1. An implantable device comprising: a hermeticpackage enclosing an electronic circuit suitable to be implanted withina head; a flexible circuit including a plurality of electrodes, eachelectrode including a plurality of common sub-electrodes, electricallycoupled to the electronic circuit suitable to contact the visual cortex;and a wireless transceiver electrically coupled to the electroniccircuit for receiving data from a transceiver external to the body thatdetermines the simulation pattern and strength for each electrode. 2.The implantable device according to claim 1, with further comprising atransmitter suitable to send data such as electrode impedance andimplant self-diagnostics wirelessly to a transceiver external to thebody.
 3. The implantable device according to claim 1, further comprisinga transmitter suitable to send data derived from neural activity at theelectrodes wirelessly to a transceiver external to the body.
 4. Theimplantable device according to claim 1, wherein the flexible circuitincludes electrodes arranged in a trapezoidal shape.
 5. The implantabledevice according to claim 1, further comprising a mounting fixtureenclosing the hermetic package and defining voids suitable to attach themounting fixture to a skull.
 6. The implantable device according toclaim 5, further defining voids suitable to accept screws for attachingthe mounting fixture to a skull.
 7. The implantable device according toclaim 5, further comprising a cover plate suitable to protect thehermetic package from impact.
 8. The implantable device according toclaim 1, further comprising a coil for sending and/or receiving data. 9.The implantable device according to claim 8, wherein the coil is on anopposite side of the hermetic package from the flexible circuit.
 10. Theimplantable device according to claim 8, wherein the coil is positionedon hermetic package 90 degrees from the flexible circuit.
 11. Theimplantable device according in to claim 1, further comprising at leastone electrode that is within a field of sub-electrodes, but not commonto the sub-electrodes.
 12. The implantable device according to claim 1,further comprising groups of sub-electrodes that are separatelycontrollable.
 13. The implantable device according to claim 1, when thesub-electrodes protrude slightly from the surface of the electrode arrayand are suitable to protrude into the cortical tissue.
 14. Theimplantable device according to claim 1, when the electrodes protrudeslightly from the surface of the electrode array and are suitable toprotrude into the cortical tissue.